1. Field of the Invention
This invention relates to an improved adhesive system for use in sealing reclosable flexible bags such as thermoplastic bags made from polyethylene. Related applications assigned to the same assignee as this application include the following copending and concurrently filed applications: "Adhesive Bag Closure That Opens Easily By Hand But Resists Opening By Contents," Ser. No. 335,800, filed Dec. 30, 1981; "Protective Strip for Z-Fold Bag Closure," Ser. No. 335,955, filed Dec. 30, 1981, now U.S. Pat. No. 4,410,130; "Manufacturing Process For Channel Seal," Ser. No. 365,814, filed Apr. 5, 1982, now U.S. Pat. No. 4,392,897; and "Adhesive Channel Closure For Flexible Bags," Ser. No. 335,798, filed Dec. 30, 1981, the last application being incorporated by reference herein.
2. Description of the Prior Art
Plastic bags are used in a variety of household applications, especially those involving packaging and storing food items, e.g., sandwiches. The use of these bags for food packing and storing appears to have added an impetus to the search for better means of sealing such bags in such a way that the bags may be opened and then resealed. One area of development has included the so-called profile bags, an example of which may be seen by referring to U.S. Pat. Re. No. 28,969 to Naito. These profile bags have at least one set of mating channels which must be placed in registration before sealing can be effected. Such profile structures may be used to construct bags which are openable and resealable. As seen in U.S. Pat. No. 4,186,786 to Kirkpatrick, colored channels may allow the user to more easily detect complete occlusion of profiled bag openings. In order to form a seal, a profile channel must be placed in registration with its mating member, but this may be inconvenient for the user.
A second area of development is the use of adhesives to form resealable closures for bags. Usually one or more strips of adhesive are applied to one surface of a bag opening and sealing is effected by contacting the surface having the adhesive strip with a second surface on the opposite side of the bag opening. For example, U.S. Pat. No. 3,670,876 to Davis discloses a bag for household use including an improved closure comprising a plurality of pressure sensitive adhesive strips located in spaced apart relationship on an exterior portion of the bag. An offset flap portion is adapted to be folded over and attached to one or more of the adhesive strips to tightly close the bag.
While bags with adhesive closures have an advantage of being adjustable to the size of the object contained in the bag, it has been difficult to find a satisfactory adhesive which is suitable for use with reclosable plastic bags, such as those made of polyethylene film, which are packaged in a carton or on a roll. More particularly, an adhesive strip which adheres well to a polyethylene surface may form a seal that is not easily reopenable without destruction of the closure or a portion of the bag, while an adhesive that forms a resealable closure may not adhere well to a base film or closure strip. One test which is generally applied to products such as this is that the product must not deteriorate when subjected to a temperature of 140.degree. F. for 24 hours.
Previous attempts at finding adhesives suitable for use in constructing resealable closures for flexible bags have involved two particular problems. First, while hot melt adhesives exhibit exceptionally high bond strengths when applied hot and are easy to apply at high speed without need of a drying step, the adhesive strength of a hot melt adhesive may build up on standing which leads to destruction of the closure upon opening. Also, the strength of a hot melt adhesive may be reduced after aging at 140.degree. F. while in contact with polyethylene film (e.g. in a sealed position with an opposing surface of polyethylene film). Second, while liquid-based adhesives such as water-dispersions of acrylates do not suffer the disabilities of strength build up or deterioration with aging at 140.degree. F. while in contact with polyethylene film, these water dispersions of adhesives do not form a satisfactorily permanent bond to polyethylene film; thus sections of an adhesive strip may be removed during opening of the bag leading to a deterioration of the seal after a number of openings and resealings. Other types of liquid-based adhesives such as those comprising organic solvents are difficult to use in a manufacturing process because of the requirement of a separate drying step and means to safely remove vapors generated thereby.
Processes have been tried to provide a closure in which the surface to which the adhesive is to be applied is pretreated so that the adhesive will more firmly adhere to the pretreated section to create an effective seal which is openable and resealable without deterioration of the seal or destruction of the closure. Pretreatment of polyolefins such as polyethylene has often involved an oxidizing step. U.S. Pat. No. 3,348,762 to Kasinkas, discloses a household bag on which an oxidized strip is formed across the upper closure flap on the surface. The oxidized strip is formed by a process in which a gap voltage generated in the air between an electrode and an area forming strip on a bag ionizes the ambient proximate thereto. The ionized ozone thus formed acts as an oxidizing agent, and oxidizes a strip across the upper closure flap on the surface beneath the electrode. A pressure sensitive adhesive is then applied to the oxidized area and dried. This type of oxidizing step, however, may be difficult to control from a manufacturing standpoint because undesired adhesion of film layers may occur, and it may be difficult to localize the area exposed to such an oxidizing step.
In the pressure sensitive adhesive art, the solution to the problem of sticking during shipment and storage has been to provide a release layer over the pressure sensitive adhesive. For example, pressure sensitive plastic tapes have a release layer which impedes the sticking of the adhesive to the plastic of the adjacent layer in the roll. This approach is not desirable for use on closures for plastic bags because of the expense and difficulty in manufacture.
Thus, it is an object of this invention to provide an adhesive system for a flexible bag which selectively adheres to one side of a closure structure. It is another object of this invention to provide a closure for a flexible bag which forms an effective seal but which is openable and resealable. It is yet another object of this invention to provide a closure for a flexible bag which closure comprises an easily manufacturable closure having an effective adhesive system. It is a further object to provide a resealable closure for a flexible bag comprising polyethylene in which the closure comprises a hot melt/acrylate adhesive system which is effective with a very thin layer of the acrylate component. These and other objects of the invention will be apparent from the following explanation.